Each object which extends Mail::Transport::Receive implement a protocol which can get messages into your application. The internals of each implementation can differ quite a lot, so have a look at each separate manual page as well.

Fatal error: the specific package (or one of its superclasses) does not implement this method where it should. This message means that some other related classes do implement this method however the class at hand does not. Probably you should investigate this and probably inform the author of the package.